/*
 * Copyright 2000-2015 JetBrains s.r.o.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.intellij.util.containers;

import com.intellij.util.ArrayUtil;
import com.intellij.util.ArrayUtilRt;
import com.intellij.util.concurrency.AtomicFieldUpdater;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;
import org.jetbrains.annotations.TestOnly;

import java.util.*;

/**
 * This class is a
 * - lock-free (CAS instead of ReentrantLock)
 * - less-memory (no lock field)
 * - less-garbage (does not create Object[0] arrays)
 * - non-cloneable, non-serializable, no-subList-method variant of {@link java.util.concurrent.CopyOnWriteArrayList}.
 * It generally is faster than COWAL in case of low write-contention.
 * (Note that it is not advisable to use COWAL in high write-contention code anyway, consider using {@link java.util.concurrent.ConcurrentHashMap}) instead)
 */
class LockFreeCopyOnWriteArrayList<E> implements List<E>, RandomAccess, ConcurrentList<E> {
    @NotNull
    private volatile Object[] array;

    LockFreeCopyOnWriteArrayList() {
        array = ArrayUtil.EMPTY_OBJECT_ARRAY;
    }

    LockFreeCopyOnWriteArrayList(@NotNull Collection<? extends E> c) {
        array = c.isEmpty() ? ArrayUtil.EMPTY_OBJECT_ARRAY : c.toArray();
    }

    @NotNull
    @TestOnly
    Object[] getArray() {
        return array;
    }

    private static final AtomicFieldUpdater<LockFreeCopyOnWriteArrayList, Object[]> ARRAY_UPDATER
            = AtomicFieldUpdater.forFieldOfType(LockFreeCopyOnWriteArrayList.class, Object[].class);

    private boolean replaceArray(@NotNull Object[] oldArray, @NotNull Object[] newArray) {
        return ARRAY_UPDATER.compareAndSet(this, oldArray, newArray);
    }

    /**
     * Returns the number of elements in this list.
     *
     * @return the number of elements in this list
     */
    @Override
    public int size() {
        return array.length;
    }

    /**
     * @return <tt>true</tt> if this list contains no elements
     */
    @Override
    public boolean isEmpty() {
        return size() == 0;
    }

    /**
     * Test for equality, coping with nulls.
     */
    private static boolean eq(Object o1, Object o2) {
        return o1 == null ? o2 == null : o1.equals(o2);
    }

    /**
     * static version of indexOf, to allow repeated calls without
     * needing to re-acquire array each time.
     *
     * @param o        element to search for
     * @param elements the array
     * @param index    first index to search
     * @param fence    one past last index to search
     * @return index of element, or -1 if absent
     */
    private static int indexOf(Object o, @NotNull Object[] elements, int index, int fence) {
        if (o == null) {
            for (int i = index; i < fence; i++) {
                if (elements[i] == null) {
                    return i;
                }
            }
        } else {
            for (int i = index; i < fence; i++) {
                if (o.equals(elements[i])) {
                    return i;
                }
            }
        }
        return -1;
    }

    /**
     * static version of lastIndexOf.
     *
     * @param o        element to search for
     * @param elements the array
     * @param index    first index to search
     * @return index of element, or -1 if absent
     */
    private static int lastIndexOf(Object o, @NotNull Object[] elements, int index) {
        if (o == null) {
            for (int i = index; i >= 0; i--) {
                if (elements[i] == null) {
                    return i;
                }
            }
        } else {
            for (int i = index; i >= 0; i--) {
                if (o.equals(elements[i])) {
                    return i;
                }
            }
        }
        return -1;
    }

    /**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested
     * @return <tt>true</tt> if this list contains the specified element
     */
    @Override
    public boolean contains(Object o) {
        Object[] elements = array;
        return indexOf(o, elements, 0, elements.length) >= 0;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public int indexOf(Object o) {
        Object[] elements = array;
        return indexOf(o, elements, 0, elements.length);
    }

    /**
     * Returns the index of the first occurrence of the specified element in
     * this list, searching forwards from <tt>index</tt>, or returns -1 if
     * the element is not found.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(i&nbsp;&gt;=&nbsp;index&nbsp;&amp;&amp;&nbsp;(e==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;e.equals(get(i))))</tt>,
     * or -1 if there is no such index.
     *
     * @param e     element to search for
     * @param index index to start searching from
     * @return the index of the first occurrence of the element in
     * this list at position <tt>index</tt> or later in the list;
     * <tt>-1</tt> if the element is not found.
     * @throws IndexOutOfBoundsException if the specified index is negative
     */
    public int indexOf(E e, int index) {
        Object[] elements = array;
        return indexOf(e, elements, index, elements.length);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public int lastIndexOf(Object o) {
        Object[] elements = array;
        return lastIndexOf(o, elements, elements.length - 1);
    }

    /**
     * Returns the index of the last occurrence of the specified element in
     * this list, searching backwards from <tt>index</tt>, or returns -1 if
     * the element is not found.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(i&nbsp;&lt;=&nbsp;index&nbsp;&amp;&amp;&nbsp;(e==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;e.equals(get(i))))</tt>,
     * or -1 if there is no such index.
     *
     * @param e     element to search for
     * @param index index to start searching backwards from
     * @return the index of the last occurrence of the element at position
     * less than or equal to <tt>index</tt> in this list;
     * -1 if the element is not found.
     * @throws IndexOutOfBoundsException if the specified index is greater
     *                                   than or equal to the current size of this list
     */
    public int lastIndexOf(E e, int index) {
        Object[] elements = array;
        return lastIndexOf(e, elements, index);
    }

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     * <p/>
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     * <p/>
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all the elements in this list
     */
    @NotNull
    @Override
    public Object[] toArray() {
        Object[] elements = array;
        if (elements.length == 0) return ArrayUtilRt.EMPTY_OBJECT_ARRAY;

        return Arrays.copyOf(elements, elements.length, Object[].class);
    }

    /**
     * Returns an array containing all of the elements in this list in
     * proper sequence (from first to last element); the runtime type of
     * the returned array is that of the specified array.  If the list fits
     * in the specified array, it is returned therein.  Otherwise, a new
     * array is allocated with the runtime type of the specified array and
     * the size of this list.
     * <p/>
     * <p>If this list fits in the specified array with room to spare
     * (i.e., the array has more elements than this list), the element in
     * the array immediately following the end of the list is set to
     * <tt>null</tt>.  (This is useful in determining the length of this
     * list <i>only</i> if the caller knows that this list does not contain
     * any null elements.)
     * <p/>
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     * <p/>
     * <p>Suppose <tt>x</tt> is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of <tt>String</tt>:
     * <p/>
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     * <p>
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing all the elements in this list
     * @throws ArrayStoreException  if the runtime type of the specified array
     *                              is not a supertype of the runtime type of every element in
     *                              this list
     * @throws NullPointerException if the specified array is null
     */
    @NotNull
    @Override
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(@NotNull T[] a) {
        Object[] elements = array;
        int len = elements.length;
        if (a.length < len) {
            return (T[]) Arrays.copyOf(elements, len, a.getClass());
        }
        System.arraycopy(elements, 0, a, 0, len);
        if (a.length > len) {
            a[len] = null;
        }
        return a;
    }

    // Positional Access Operations

    @SuppressWarnings("unchecked")
    private E get(@NotNull Object[] a, int index) {
        return (E) a[index];
    }

    /**
     * {@inheritDoc}
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public E get(int index) {
        return get(array, index);
    }

    /**
     * Replaces the element at the specified position in this list with the
     * specified element.
     */
    @Override
    public E set(int index, E element) throws IndexOutOfBoundsException {
        E oldValue;
        Object[] elements;
        Object[] newElements;
        do {
            elements = array;
            oldValue = get(elements, index);

            if (oldValue == element) {
                // Not quite a no-op; ensures volatile write semantics
                newElements = elements;
            } else {
                newElements = createArraySet(elements, index, element);
            }
        } while (!replaceArray(elements, newElements));
        return oldValue;
    }

    @NotNull
    private static Object[] createArraySet(@NotNull Object[] elements, int index, Object element) {
        int len = elements.length;
        Object[] newElements = Arrays.copyOf(elements, len, Object[].class);
        newElements[index] = element;
        return newElements;
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    @Override
    public boolean add(E e) {
        while (true) {
            Object[] elements = array;
            Object[] newElements = createArrayAdd(elements, e);
            if (replaceArray(elements, newElements)) break;
        }
        return true;
    }

    @NotNull
    private Object[] createArrayAdd(@NotNull Object[] elements, E e) {
        int len = elements.length;
        Object[] newElements = new Object[len + 1];
        if (len != 0) {
            System.arraycopy(elements, 0, newElements, 0, len);
        }
        newElements[len] = e;
        return newElements;
    }

    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     */
    @Override
    public void add(int index, E element) throws IndexOutOfBoundsException {
        while (true) {
            Object[] elements = array;
            Object[] newElements = createArrayAdd(elements, index, element);
            if (replaceArray(elements, newElements)) break;
        }
    }

    @NotNull
    private Object[] createArrayAdd(@NotNull Object[] elements, int index, E element) {
        int len = elements.length;
        if (index > len || index < 0) {
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + len);
        }
        Object[] newElements = new Object[len + 1];
        if (index != 0) {
            System.arraycopy(elements, 0, newElements, 0, index);
        }
        int numMoved = len - index;
        if (numMoved != 0) {
            System.arraycopy(elements, index, newElements, index + 1, numMoved);
        }
        newElements[index] = element;
        return newElements;
    }

    /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).  Returns the element that was removed from the list.
     */
    @Override
    public E remove(int index) throws IndexOutOfBoundsException {
        E oldValue;
        while (true) {
            Object[] elements = array;
            Object[] newElements = createArrayRemove(elements, index);
            if (replaceArray(elements, newElements)) {
                oldValue = get(elements, index);
                break;
            }
        }
        return oldValue;
    }

    @NotNull
    private static Object[] createArrayRemove(@NotNull Object[] elements, int index) {
        int len = elements.length;
        Object[] newElements = len == 1 ? ArrayUtilRt.EMPTY_OBJECT_ARRAY : new Object[len - 1];
        if (index != 0) {
            System.arraycopy(elements, 0, newElements, 0, index);
        }
        int numMoved = len - index - 1;
        if (numMoved != 0) {
            System.arraycopy(elements, index + 1, newElements, index, numMoved);
        }
        return newElements;
    }

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If this list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    @Override
    public boolean remove(Object o) {
        while (true) {
            Object[] elements = array;
            Object[] newElements = createArrayRemove(elements, o);
            if (newElements == null) {
                return false;
            }
            if (replaceArray(elements, newElements)) return true;
        }
    }

    // null means not found
    @Nullable
    private static Object[] createArrayRemove(@NotNull Object[] elements, Object o) {
        int len = elements.length;
        if (len == 0) {
            return null;
        }
        // Copy while searching for element to remove
        // This wins in the normal case of element being present
        int newLen = len - 1;
        Object[] newElements = newLen == 0 ? ArrayUtilRt.EMPTY_OBJECT_ARRAY : new Object[newLen];

        int i;
        for (i = newLen; i != 0; --i) {
            Object element = elements[i];
            if (eq(o, element)) {
                // found one;  copy remaining and exit
                System.arraycopy(elements, 0, newElements, 0, i);
                break;
            }
            newElements[i - 1] = element;
        }

        // special handling for last cell
        if (i == 0 && !eq(o, elements[0])) {
            return null;
        }
        return newElements;
    }

    /**
     * Removes from this list all of the elements whose index is between
     * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.
     * Shifts any succeeding elements to the left (reduces their index).
     * This call shortens the list by <tt>(toIndex - fromIndex)</tt> elements.
     * (If <tt>toIndex==fromIndex</tt>, this operation has no effect.)
     *
     * @param fromIndex index of first element to be removed
     * @param toIndex   index after last element to be removed
     * @throws IndexOutOfBoundsException if fromIndex or toIndex out of range
     *                                   ({@code {fromIndex < 0 || toIndex > size() || toIndex < fromIndex})
     */
    private void removeRange(int fromIndex, int toIndex) {
        Object[] elements;
        Object[] newElements;
        do {
            elements = array;
            int len = elements.length;

            if (fromIndex < 0 || toIndex > len || toIndex < fromIndex) {
                throw new IndexOutOfBoundsException();
            }
            int newlen = len - (toIndex - fromIndex);
            int numMoved = len - toIndex;
            if (numMoved == 0) {
                newElements = Arrays.copyOf(elements, newlen, Object[].class);
            } else {
                newElements = new Object[newlen];
                System.arraycopy(elements, 0, newElements, 0, fromIndex);
                System.arraycopy(elements, toIndex, newElements, fromIndex, numMoved);
            }
        }
        while (!replaceArray(elements, newElements));
    }

    /**
     * Append the element if not present.
     *
     * @param e element to be added to this list, if absent
     * @return <tt>true</tt> if the element was added
     */
    @Override
    public boolean addIfAbsent(E e) {
        Object[] elements;
        Object[] newElements;
        do {
            // Copy while checking if already present.
            // This wins in the most common case where it is not present
            elements = array;
            int len = elements.length;
            newElements = new Object[len + 1];
            for (int i = 0; i < len; ++i) {
                if (eq(e, elements[i])) {
                    return false; // exit, throwing away copy
                }
                newElements[i] = elements[i];
            }
            newElements[len] = e;
        }
        while (!replaceArray(elements, newElements));
        return true;
    }

    /**
     * Returns <tt>true</tt> if this list contains all of the elements of the
     * specified collection.
     *
     * @param c collection to be checked for containment in this list
     * @return <tt>true</tt> if this list contains all of the elements of the
     * specified collection
     * @throws NullPointerException if the specified collection is null
     * @see #contains(Object)
     */
    @Override
    public boolean containsAll(@NotNull Collection<?> c) {
        Object[] elements = array;
        int len = elements.length;
        for (Object e : c) {
            if (indexOf(e, elements, 0, len) < 0) {
                return false;
            }
        }
        return true;
    }

    /**
     * Removes from this list all of its elements that are contained in
     * the specified collection. This is a particularly expensive operation
     * in this class because of the need for an internal temporary array.
     *
     * @param c collection containing elements to be removed from this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws ClassCastException   if the class of an element of this list
     *                              is incompatible with the specified collection
     *                              (<a href="../Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *                              specified collection does not permit null elements
     *                              (<a href="../Collection.html#optional-restrictions">optional</a>),
     *                              or if the specified collection is null
     * @see #remove(Object)
     */
    @Override
    public boolean removeAll(@NotNull Collection<?> c) {
        if (c.isEmpty()) return false;
        while (true) {
            Object[] elements = array;
            Object[] newElements = createArrayRemoveAll(elements, c);
            if (newElements == null) return false;
            if (replaceArray(elements, newElements)) return true;
        }
    }

    // null means not found
    @Nullable
    private static Object[] createArrayRemoveAll(@NotNull Object[] elements, @NotNull Collection<?> c) {
        int len = elements.length;
        if (len == 0) {
            return null;
        }
        // temp array holds those elements we know we want to keep
        int newLen = 0;
        Object[] temp = new Object[len];
        for (Object element : elements) {
            if (!c.contains(element)) {
                temp[newLen++] = element;
            }
        }
        if (newLen == len) {
            return null;
        }
        return Arrays.copyOf(temp, newLen, Object[].class);
    }

    /**
     * Retains only the elements in this list that are contained in the
     * specified collection.  In other words, removes from this list all of
     * its elements that are not contained in the specified collection.
     *
     * @param c collection containing elements to be retained in this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws ClassCastException   if the class of an element of this list
     *                              is incompatible with the specified collection
     *                              (<a href="../Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *                              specified collection does not permit null elements
     *                              (<a href="../Collection.html#optional-restrictions">optional</a>),
     *                              or if the specified collection is null
     * @see #remove(Object)
     */
    @Override
    public boolean retainAll(@NotNull Collection<?> c) {
        while (true) {
            Object[] elements = array;
            Object[] newElements = createArrayRetainAll(elements, c);
            if (newElements == null) return false;
            if (replaceArray(elements, newElements)) return true;
        }
    }

    @Nullable
    private static Object[] createArrayRetainAll(@NotNull Object[] elements, @NotNull Collection<?> c) {
        int len = elements.length;
        if (len == 0) {
            return null;
        }
        // temp array holds those elements we know we want to keep
        int newlen = 0;
        Object[] temp = new Object[len];
        for (Object element : elements) {
            if (c.contains(element)) {
                temp[newlen++] = element;
            }
        }
        if (newlen == len) {
            return null;
        }
        return Arrays.copyOf(temp, newlen, Object[].class);
    }

    /**
     * Appends all of the elements in the specified collection that
     * are not already contained in this list, to the end of
     * this list, in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param c collection containing elements to be added to this list
     * @return the number of elements added
     * @throws NullPointerException if the specified collection is null
     * @see #addIfAbsent(Object)
     */
    @Override
    public int addAllAbsent(@NotNull Collection<? extends E> c) {
        Object[] cs = c.toArray();
        if (cs.length == 0) {
            return 0;
        }

        Object[] uniq = new Object[cs.length];
        Object[] elements;
        Object[] newElements;
        int added;
        do {
            elements = array;
            added = 0;
            int len = elements.length;
            for (Object e : cs) { // scan for duplicates
                if (indexOf(e, elements, 0, len) < 0 &&
                        indexOf(e, uniq, 0, added) < 0) {
                    uniq[added++] = e;
                }
            }
            if (added == 0) {
                return 0;
            }
            newElements = Arrays.copyOf(elements, len + added, Object[].class);
            System.arraycopy(uniq, 0, newElements, len, added);
        }
        while (!replaceArray(elements, newElements));
        return added;
    }

    /**
     * Removes all of the elements from this list.
     * The list will be empty after this call returns.
     */
    @Override
    public void clear() {
        array = ArrayUtilRt.EMPTY_OBJECT_ARRAY;
    }

    /**
     * Appends all of the elements in the specified collection to the end
     * of this list, in the order that they are returned by the specified
     * collection's iterator.
     *
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     * @see #add(Object)
     */
    @Override
    public boolean addAll(@NotNull Collection<? extends E> c) {
        if (c.isEmpty()) return false;
        Object[] cs = c.toArray();
        if (cs.length == 0) {
            return false;
        }

        while (true) {
            Object[] elements = array;
            Object[] newElements = createArrayAddAll(elements, cs);
            if (replaceArray(elements, newElements)) return true;
        }
    }

    @NotNull
    private static Object[] createArrayAddAll(@NotNull Object[] elements, @NotNull Object[] cs) {
        int len = elements.length;
        Object[] newElements = Arrays.copyOf(elements, len + cs.length, Object[].class);
        System.arraycopy(cs, 0, newElements, len, cs.length);
        return newElements;
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in this list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element
     *              from the specified collection
     * @param c     collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     * @see #add(int, Object)
     */
    @Override
    public boolean addAll(int index, @NotNull Collection<? extends E> c) throws IndexOutOfBoundsException {
        Object[] cs = c.toArray();
        if (cs.length == 0) {
            return false;
        }

        while (true) {
            Object[] elements = array;
            Object[] newElements = createArrayAddAll(elements, index, cs);
            if (replaceArray(elements, newElements)) break;
        }

        return true;
    }

    @NotNull
    private static Object[] createArrayAddAll(@NotNull Object[] elements, int index, @NotNull Object[] cs) {
        int len = elements.length;
        if (index > len || index < 0) {
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + len);
        }
        int numMoved = len - index;
        Object[] newElements;
        if (numMoved == 0) {
            newElements = Arrays.copyOf(elements, len + cs.length, Object[].class);
        } else {
            newElements = new Object[len + cs.length];
            System.arraycopy(elements, 0, newElements, 0, index);
            System.arraycopy(elements, index, newElements, index + cs.length, numMoved);
        }
        System.arraycopy(cs, 0, newElements, index, cs.length);
        return newElements;
    }

    /**
     * Returns a string representation of this list.  The string
     * representation consists of the string representations of the list's
     * elements in the order they are returned by its iterator, enclosed in
     * square brackets (<tt>"[]"</tt>).  Adjacent elements are separated by
     * the characters <tt>", "</tt> (comma and space).  Elements are
     * converted to strings as by {@link String#valueOf(Object)}.
     *
     * @return a string representation of this list
     */
    @Override
    @NotNull
    public String toString() {
        return Arrays.toString(array);
    }

    /**
     * Compares the specified object with this list for equality.
     * Returns {@code true} if the specified object is the same object
     * as this object, or if it is also a {@link List} and the sequence
     * of elements returned by an {@linkplain List#iterator() iterator}
     * over the specified list is the same as the sequence returned by
     * an iterator over this list.  The two sequences are considered to
     * be the same if they have the same length and corresponding
     * elements at the same position in the sequence are <em>equal</em>.
     * Two elements {@code e1} and {@code e2} are considered
     * <em>equal</em> if {@code (e1 == null ? e2 == null : e1.equals(e2))}.
     *
     * @param o the object to be compared for equality with this list
     * @return {@code true} if the specified object is equal to this list
     */
    @Override
    public boolean equals(Object o) {
        if (o == this) {
            return true;
        }
        if (!(o instanceof List)) {
            return false;
        }

        List<?> list = (List<?>) o;
        Iterator<?> it = list.iterator();
        for (Object element : array) {
            if (!it.hasNext() || !eq(element, it.next())) {
                return false;
            }
        }
        return !it.hasNext();
    }

    /**
     * Returns the hash code value for this list.
     * <p/>
     * <p>This implementation uses the definition in {@link List#hashCode}.
     *
     * @return the hash code value for this list
     */
    @Override
    public int hashCode() {
        int hashCode = 1;
        for (Object obj : array) {
            hashCode = 31 * hashCode + (obj == null ? 0 : obj.hashCode());
        }
        return hashCode;
    }

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     * <p/>
     * <p>The returned iterator provides a snapshot of the state of the list
     * when the iterator was constructed. No synchronization is needed while
     * traversing the iterator. The iterator does <em>NOT</em> support the
     * <tt>remove</tt> method.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    @NotNull
    @Override
    public Iterator<E> iterator() {
        Object[] elements = array;
        if (elements.length == 0) return EmptyIterator.getInstance();

        return new COWIterator(elements, 0);
    }

    /**
     * {@inheritDoc}
     * <p/>
     * <p>The returned iterator provides a snapshot of the state of the list
     * when the iterator was constructed. No synchronization is needed while
     * traversing the iterator. The iterator does <em>NOT</em> support the
     * <tt>remove</tt>, <tt>set</tt> or <tt>add</tt> methods.
     */
    @NotNull
    @Override
    public ListIterator<E> listIterator() {
        return listIterator(0);
    }

    /**
     * {@inheritDoc}
     * <p/>
     * <p>The returned iterator provides a snapshot of the state of the list
     * when the iterator was constructed. No synchronization is needed while
     * traversing the iterator. The iterator does <em>NOT</em> support the
     * <tt>remove</tt>, <tt>set</tt> or <tt>add</tt> methods.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @NotNull
    @Override
    public ListIterator<E> listIterator(final int index) {
        Object[] elements = array;
        int len = elements.length;
        if (index < 0 || index > len) {
            throw new IndexOutOfBoundsException("Index: " + index);
        }

        return elements.length == 0 ? EmptyListIterator.<E>getInstance() : new COWIterator(elements, index);
    }

    private class COWIterator implements ListIterator<E> {
        /**
         * Snapshot of the array
         */
        private final Object[] snapshot;
        /**
         * Index of element to be returned by subsequent call to next.
         */
        private int cursor;
        private int lastRet = -1; // index of last element returned; -1 if no such

        private COWIterator(@NotNull Object[] elements, int initialCursor) {
            cursor = initialCursor;
            snapshot = elements;
        }

        @Override
        public boolean hasNext() {
            return cursor < snapshot.length;
        }

        @Override
        public boolean hasPrevious() {
            return cursor > 0;
        }

        @Override
        @SuppressWarnings("unchecked")
        public E next() {
            if (!hasNext()) {
                throw new NoSuchElementException();
            }
            lastRet = cursor;
            return (E) snapshot[cursor++];
        }

        @Override
        @SuppressWarnings("unchecked")
        public E previous() {
            if (!hasPrevious()) {
                throw new NoSuchElementException();
            }
            return (E) snapshot[lastRet = --cursor];
        }

        @Override
        public int nextIndex() {
            return cursor;
        }

        @Override
        public int previousIndex() {
            return cursor - 1;
        }

        @Override
        public void remove() {
            if (lastRet < 0) {
                throw new NoSuchElementException();
            }
            @SuppressWarnings("unchecked")
            E e = (E) snapshot[lastRet];
            lastRet = -1;
            LockFreeCopyOnWriteArrayList.this.remove(e);
        }

        @Override
        public void set(E e) {
            throw new UnsupportedOperationException();
        }

        @Override
        public void add(E e) {
            throw new UnsupportedOperationException();
        }
    }

    @NotNull
    @Override
    public List<E> subList(int fromIndex, int toIndex) {
        throw new UnsupportedOperationException();
    }
}
